darling-libffi/darwin

These header files potentially need to be updated whenever we update to a newer version of upstream libffi.

Running the generate-darwin-source-and-headers.py script in the parent directory will create the updated headers
in darwin_ios/include and darwin_osx/include.  Replace the headers in darwin/include with newer copies of those
headers.

fficonfig*.h don't get installed, so just copy the _arch.h versions in place.

ffitarget*.h are customized per architecture, so just copy the _arch.h versions in place.  Note that i386 and
x86_64 are the same, so just squash them.

The only difference between the ffi_*.h are in the architecture definition macros, so it's easiest to just
hand craft those as necessary.  It's actually probably easier to compare the differences in ffi.h.in between
versions and applying that to our ffi.h.

Don't forget to add availability markup as needed.